Chucks of the kind set forth in the opening part of this specification are also referred to as four-jaw chucks or four-jaw clamping chucks. Chucks for centric clamping are basically required whenever workpieces have to be clamped in machine tools for machining, in particular when they are machined in rotational mode. In modern manufacturing environments there is increasingly a need for clamping devices which can be used both for turning and also for milling. Accordingly the invention concerns a chuck for such purposes of use.
The reason for at all considering the use of four jaw chucks in comparison with three-jaw chucks is that, with a larger number of jaws which embracingly grip the workpiece, the spot load on each individual jaw is reduced. Consequently, with a larger number of jaws, the forces acting on the workpiece and consequently deforming the workpiece are more uniformly distributed around the periphery of the workpiece. That increases the dimensional accuracy of the workpiece after machining.
Four-jaw chucks are known from the state of the art, which provide for centric clamping. These however generally suffer from the disadvantage that they are only suitable for clamping round workpieces or have to be of a mirror-symmetrical configuration in both clamping jaw planes so that all four jaws can engage the workpiece when clamping it. By virtue of a lack of compensation, in respect of the workpiece geometry, such rigidly centrically clamping chucks involve only limited practical use in relation to “non-round” workpieces.
Four-jaw compensation chucks are also known from the state of the art, which however also suffer from crucial disadvantages. In the case of many four-jaw chucks with a compensation function the reproduction accuracy in terms of centric clamping is restricted.
WO2011/137884 A1 discloses for example a four jaw chuck having a compensation function in such a way that adjacent jaws of the chuck are coupled together by way of one or more rocker bars. If, when dealing with a workpiece which is not rotationally symmetrical, firstly a first pair of jaws is brought into contact with the workpiece while the other pair of jaws is not yet in contact therewith, the corresponding rocker bars are deflected and the second pair of jaws successively approaches the workpiece. That kinematic solution suffers from the disadvantage that, by virtue of the lever lengths which necessarily have to be tolerated between the drive force and the jaw there is a certain elasticity in the system and drive force losses occur. The high complexity in terms of the parts involved also makes the systems potentially susceptible to trouble.
A further approach for providing for compensation in four jaw chucks is to be found in the field of chucks which are actuated by power clamping. DE 10 2004 001 839 A1 discloses a four-jaw chuck in which the four chucks are pulled along conical slide surfaces by a clamping cylinder. A hydraulic or mechanical power deflection system leads to the displacement of conical compensation elements in opposite relationship to the clamping direction, which are intended to permit jaws which are not yet in contact with the workpiece to approach same. That system is viewed in particular as suffering from the disadvantage that the maximum possible compensation travel is extremely slight and the structural size of the clamping device overall is unacceptably high for certain purposes of use. In addition the system is again suitable for hand chucking operation.
The above-described disadvantages are addressed in DE 10 2013 201 231 B3. Described therein are four jaw chucks having a compensation function, which in some embodiments are designed for hand chucking operation but also for power chucking actuation. While the chucks presented therein have unlimitedly proven themselves in practice and afford a compensation function with a large compensation range with maintained centricity in the clamping operation there is nonetheless still a need to develop the existing technology. In particular there is a wish to develop a chuck which is more economical in manufacture and which as far as possible affords improved power transmission from the drive unit to the base jaws.